$ tree
.
├── app
│   └── widgets
│       ├── components
│       │   └── login_button.dart
│       └── screen
│           ├── camera.dart
│           ├── camera_display.dart
│           └── login.dart
└── main.dart

import 'package:flutter/material.dart';
import 'package:test_app/app/widgets/screen/login.dart';
import 'package:test_app/app/widgets/screen/camera.dart';

void main() {
  // この一文を入れないとエラーになる
  WidgetsFlutterBinding.ensureInitialized();
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
        title: 'TEST',
        theme: ThemeData(
          primaryColor: Colors.blueGrey[800],
          visualDensity: VisualDensity.adaptivePlatformDensity,
        ),
        home: CameraHome(),
        routes: <String, WidgetBuilder>{
          '/camera': (BuildContext context) => CameraHome(),
          '/login': (BuildContext context) => LoginPage(title: 'firebase login'),
        });
  }
}

// firebase auth
import 'package:flutter/material.dart';
import 'package:firebase_auth/firebase_auth.dart';
import 'package:google_sign_in/google_sign_in.dart';

class LoginPage extends StatefulWidget {
  LoginPage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _LoginPageState createState() => _LoginPageState();
}

class _LoginPageState extends State<LoginPage> {
  final GoogleSignIn _googleSignIn = GoogleSignIn();
  final FirebaseAuth _auth = FirebaseAuth.instance;

  Future<User> _handleSignIn() async {
    GoogleSignInAccount googleCurrentUser = _googleSignIn.currentUser;
    try {
      if (googleCurrentUser == null)
        googleCurrentUser = await _googleSignIn.signInSilently();
      if (googleCurrentUser == null)
        googleCurrentUser = await _googleSignIn.signIn();
      if (googleCurrentUser == null) return null;

      GoogleSignInAuthentication googleAuth =
          await googleCurrentUser.authentication;
      final AuthCredential credential = GoogleAuthProvider.credential(
        accessToken: googleAuth.accessToken,
        idToken: googleAuth.idToken,
      );
      final User user = (await _auth.signInWithCredential(credential)).user;
      print("signed in " + user.displayName);

      return user;
    } catch (e) {
      print(e);
      return null;
    }
  }

  @override
  Widget build(BuildContext context) {
    return WillPopScope(
        onWillPop: () {
          Navigator.of(context).pop("戻るアイコンからの遷移");
          return Future.value(false);
        },
        child: Scaffold(
          appBar: AppBar(
            title: Text(widget.title),
          ),
          body: Center(
            child: Column(
                mainAxisAlignment: MainAxisAlignment.center,
                children: <Widget>[
                  RaisedButton(
                    child: Text('Sign in Google'),
                    onPressed: () {
                      _handleSignIn()
                          .then((User user) => Navigator.of(context).pop("戻るアイコンからの遷移"))
                          .catchError((e) => print(e));
                    },
                  ),
                ]),
          ),
        ));
  }
}

import 'package:flutter/material.dart';
import 'package:camera/camera.dart';
import 'package:path/path.dart' show join;
import 'package:path_provider/path_provider.dart';

import 'package:test_app/app/widgets/screen/camera_display.dart';
import 'package:test_app/app/widgets/components/login_button.dart';

class CameraHome extends StatefulWidget {
  // final CameraDescription camera;

  // const CameraHome({Key key, @required this.camera}) : super(key: key);
  const CameraHome({Key key}) : super(key: key);

  @override
  State<StatefulWidget> createState() => CameraHomeState();
}

class CameraHomeState extends State<CameraHome> {
  // デバイスのカメラを制御するコントローラ
  CameraController _cameraController;

  LoginButton loginButton = new LoginButton();

  var action;

  // コントローラーに設定されたカメラを初期化する関数
  Future<void> _initializeCameraController() async {
    // デバイスで使用可能なカメラの一覧を取得する
    final cameras = await availableCameras();

    // 利用可能なカメラの一覧から、指定のカメラを取得する
    final firstCamera = cameras.first;

    // コントローラを初期化
    _cameraController = CameraController(
        // 使用するカメラをコントローラに設定
        firstCamera,
        // 使用する解像度を設定
        // low : 352x288 on iOS, 240p (320x240) on Android
        // medium : 480p (640x480 on iOS, 720x480 on Android)
        // high : 720p (1280x720)
        // veryHigh : 1080p (1920x1080)
        // ultraHigh : 2160p (3840x2160)
        // max : 利用可能な最大の解像度
        ResolutionPreset.max);
    // コントローラーに設定されたカメラを初期化
    return _cameraController.initialize();
  }

  @override
  void initState() {
    super.initState();
  }

  @override
  void dispose() {
    // ウィジェットが破棄されたタイミングで、カメラのコントローラを破棄する
    _cameraController.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('カメラ撮影画面'),
        actions: [loginButton],
      ),
      // FutureBuilderを実装
      body: FutureBuilder<void>(
        future: _initializeCameraController(),
        builder: (context, snapshot) {
          if (snapshot.connectionState == ConnectionState.done) {
            // カメラの初期化が完了したら、プレビューを表示
            return CameraPreview(_cameraController);
          } else {
            // カメラの初期化中はインジケーターを表示
            return const Center(child: CircularProgressIndicator());
          }
        },
      ),
      floatingActionButton: Column(
        verticalDirection: VerticalDirection.up, // childrenの先頭を下に配置
        mainAxisSize: MainAxisSize.min,
        children: <Widget>[
          FloatingActionButton(
            child: const Icon(Icons.camera_alt),
            heroTag: 'camera',
            // ボタンが押下された際の処理
            onPressed: () async {
              try {
                // 画像を保存するパスを作成する
                final path = join(
                  (await getApplicationDocumentsDirectory()).path,
                  '${DateTime.now()}.png',
                );
　
                // カメラで画像を撮影する
                await _cameraController.takePicture(path);
　
                // 画像を表示する画面に遷移
                Navigator.push(
                  context,
                  MaterialPageRoute(
                    builder: (context) => CameraDisplay(imgPath: path),
                  ),
                );
              } catch (e) {
                print(e);
              }
            },
          ),
          Container(
            // 余白のためContainerでラップ
            margin: EdgeInsets.only(bottom: 16.0),
            child: FloatingActionButton(
              child: const Icon(Icons.settings),
              heroTag: 'setting',
              onPressed: () {
                showDialog(
                  context: context,
                  builder: (context) {
                    return AlertDialog(
                      title: Text('サンプルダイアログ'),
                      content: Text('アプリを閉じますか'),
                      actions: <Widget>[
                        FlatButton(
                          child: Text("CANCEL"),
                          onPressed: () => Navigator.pop(context),
                        ),
                        FlatButton(
                          child: Text("OK"),
                          onPressed: () => Navigator.pop(context),
                        ),
                      ],
                    );
                  },
                );
              },
            ),
          ),
        ],
      ),
    );
  }
}

import 'package:flutter/material.dart';
import 'dart:io';

class CameraDisplay extends StatelessWidget {
  // 表示する画像のパス
  final String imgPath;

  // 画面のコンストラクタ
  const CameraDisplay({Key key, this.imgPath}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: const Text('Picture'),
        ),
        body: Column(
          // Imageウィジェットで画像を表示する
          children: [Expanded(child: Image.file(File(imgPath)))],
        )
    );
  }
}

import 'package:flutter/material.dart';

import 'package:firebase_core/firebase_core.dart';
import 'package:firebase_auth/firebase_auth.dart';
import 'package:google_sign_in/google_sign_in.dart';

class LoginButton extends StatefulWidget {
  final LoginButtonState abs = LoginButtonState();

  void refresh() {
    debugPrint("abs refresh");
    abs.refresh();
  }

  @override
  LoginButtonState createState() => abs;
}

class LoginButtonState extends State<LoginButton> {

  final GoogleSignIn _googleSignIn = GoogleSignIn();

  void refresh() {
    debugPrint("refresh");
    setState(() {});
  }

  Future<void> _handleSignOut() async {
    debugPrint("sign out");
    try {
      await FirebaseAuth.instance.signOut();
      await _googleSignIn.signOut();
    } catch (e) {
      debugPrint(e);
    }
    refresh();
  }

  @override
  Widget build(BuildContext context) {
    return FutureBuilder(
        // Initialize FlutterFire:
        future: Firebase.initializeApp(),
        builder: (context, snapshot) {
          if (snapshot.hasError) {
            debugPrint(snapshot.error.toString());
            return Center(
                child: Text(
              snapshot.error.toString(),
              textDirection: TextDirection.ltr,
            ));
          }

          // Once complete, show your application
          if (snapshot.connectionState == ConnectionState.done) {
            final User currentUser = FirebaseAuth.instance.currentUser;
            if (currentUser == null) {
              return IconButton(
                icon: Icon(Icons.login),
                onPressed: () async {
                  final result = await Navigator.pushNamed(context, "/login");
                  refresh();
                },
              );
            } else {
              return IconButton(
                icon: Icon(Icons.logout),
                onPressed: () {
                  _handleSignOut().catchError((e) => print(e));
                },
              );
            }
          }
          // Otherwise, show something whilst waiting for initialization to complete
          return Center(
              child: Text(
                'Loading ...',
                textDirection: TextDirection.ltr,
              ));
        });
  }
}